1. Field of the Invention
This invention relates to generation of acoustic waves and more particularly to devices for enhancing the efficiency of propagation of acoustic waves by an energy-acoustic transducer utilizing the principle of thermoelastic expansion.
2. Description of the Prior Art
U.S Pat. No. 3,532,181 of De Maria et al. entitled "Laser Induced Acoustic Generator" teaches directing a pulse train from a laser onto a thin absorbing film which is preferably bonded or deposited on an acoustic transmitting medium such as a crystalline bar. The film may be sandwiched between two crystals. A single crystal material is preferred, and use of a liquid cell is mentioned. The thin film is preferably of Au, SN, Cu or Ag. No mention is made of the fact that it is desirable to clamp the film acoustically to enhance the performance of the device.
"Generation of Elastic Waves by Transient Surface Heating" by R. M. White, Journal of Applied Physics, Vol. 34, No. 12, December 1963, pp. 3559-3567 teaches at p. 3563 the theoretical concept of constraining the surface of the energy absorbing material which is discussed mathematically. A special case is described corresponding to a "constrained surface in which two identical semi-infinite bodies meet at a plane where they are welded together and where energy absorption takes place . . . . This mathematical model may be used to simulate the absorption of r.f. energy at the joint between two transparent media which are slightly absorptive near their common boundary." There is no suggestion of any practical way in which any amplitude enhancement can be achieved by constraining the boundary of the energy absorbing layer. Hence, this reference teaches no method of using the effect.
"Laser Excitation of Microwave Sound in Solids" by Cachier, Journal of the Acoustical Society of America, Vol. 49, No. 3 (Part 3), pp. 974-978 shows on page 977 a titanium layer of 500 Angstroms thickness which is absorptive of all light deposited over a gold film of 1 micron thickness which in turn is deposited upon a 1000 Angstroms thick titanium film deposited upon a crystal delay line. The first layer of titanium is a laser energy absorbing layer, and it is exposed to the atmosphere without any constraint.
U.S. Pat. No. 3,322,231 of Gournay shows a laser source aimed through a glass window in a clamping plate into a liquid (water) to generate seismic pulses. In this case, no intermediate absorptive layer is employed to convert light energy to acoustic energy. Furthermore, no highly absorptive material is employed because water is only slightly absorptive of the light being used. Instead, the transparent body of water itself is required to absorb the energy. The glass and the plate which can be quite extensive are employed to provide a clamp of the upper surface of the fluid. The use of concentrated electromagnetic energy is required to provide high output. Even any highly pigmented liquid would not produce high absorption as it would have a relatively large optical absorption length yielding a small optical energy absorption. "Multichannel, Random-Access Acoustic Echo Storage Memory" by Melcher et al., the inventors hereof, in the IBM Technical Disclosure Bulletin, Vol. 18, No. 7, December 1975, pp. 2362-3, teaches provision of a block of solid material coated with laser energy absorbing thin film segments which are coated with a transparent overlay to shorten thermal relaxation times.
All of the above prior art shows that while many of the elements of the instant invention have been available in the prior art, they have not been combined heretofore to provide the improved results which are achieved by employing the present invention.